1. Field of the Invention
The present invention relates to a hydrostatic transmission apparatus which is mounted to a working vehicle such as a tractor or a wheel loader, the apparatus comprising a hydraulic pump and a hydraulic motor.
2. Description of the Related Art
As disclosed in Japanese Patent Application Laid-Open No. 11-59210 (1999), a conventional hydrostatic transmission apparatus generally has a structure consisting of one hydraulic motor and one hydraulic pump, namely, a so-called one pump and one motor type structure.
Normally, the one pump and one motor type hydrostatic transmission apparatus (hereinafter, referred to as an HST) comprises a fixed displacement hydraulic motor and a variable displacement hydraulic pump, and characteristics of an output rotation of the HST and an output torque are determined by the displacements of the hydraulic pump and the hydraulic motor. Namely, the output torque of the hydraulic motor at the time of performing a lowest speed operation cannot exceed a predetermined load pressure (maximum circuit pressure by a relief valve).
For example, the maximum torque of the HST is desired to be strengthened depending on types or the like of an applied working vehicle. As a countermeasure against this case, firstly enlargement of the hydraulic motor and to make displacement of the hydraulic motor variable are considered. In enlargement of the hydraulic motor, namely, means for increasing the displacement of the hydraulic motor (heighten a reducing ratio) should be enlarged in accordance with the displacement of the hydraulic pump. For this reason, this has a difficulty in that reconstruction of an entire hydraulic system is necessary. Moreover, in means in which the hydraulic motor is made to be of the variable displacement type and thus lower speed rotation becomes possible and the torque is heightened, there arises a problem that the reducing ratio as the HST is limited due to a permissible rotating speed of the hydraulic motor and it is difficult to satisfy an initial reducing ratio area. As explained above, any means admit further improvement.